The present invention relates to a thermal load reducing system for a nuclear reactor vessel, available for the reduction of thermal load near a coolant liquid surface of a reactor vessel and for the reduction of thermal load near a temperature stratified layer in a reactor vessel.
A reactor vessel in a fast breeder reactor is supported at its upper end by a concrete wall, which must be maintained at the temperature of 100° C. or lower. Because it has a high-temperature coolant at 550° C. or higher in a plenum above reactor core, there occurs a steep temperature gradient in the vertical direction from the coolant liquid surface to the upper supported end. In particular, during starting operation, both temperature and liquid level rise at the same time, the gradient becomes steeper. As a result, high thermal stress develops, in principle, on the reactor wall near the liquid surface where the temperature gradient deflects.
To cope with this problem, attempts have been made in the past to prevent the rise of liquid level using a liquid level controller, to evenly cool down the reactor wall using a reactor wall cooling system, and to reduce the bending stress by designing in a thin-wall structure. Also proposed is a reduction method of the temperature gradient near the liquid surface, arranging a liner to form a heat insulated space from reactor vessel wall below the coolant liquid surface to directly below the seal plug, in cooperation with reactor vessel, and by filling heat insulating material into the heat insulated space (Japanese Publication Number JP-A-57-80594).
As described above, the conventional method for reducing thermal load has its principal aims to prevent the rise of liquid level using a liquid level controller, to evenly cool down the reactor wall using a reactor wall cooling system, and to decrease bending stress by designing in a thin-wall structure. The liquid level controller and the reactor wall cooling system result in higher cost because of the increase of system components. For designing the system in a thin-wall structure, there was a limitation due to the possibility of other failure modes. The method described in the patent document referred to above, also leads to higher cost due to the increase of system components.